Heart-rending images of emaciated sea lion pups hauling ashore on Southern California beaches have captured public attention, but scientists say the strandings may be part of the ebb and flow of the sea lion population and a result of its newly restored abundance.

Sea lion numbers have skyrocketed in recent years, spelling success for the species, but leaving the animals vulnerable to natural fluctuations in prey. The key factor in recent deaths could be ocean changes that have left sea lion mothers too malnourished to nurse their young, according to a new study.

Researchers have suspected that a crash in prey fish triggered the waves of strandings that began in 2013 and spiked last year. The new study, published last month in the Royal Society Open Science, found a scarcity of the fatty, high-calorie sardines and anchovies the animals prefer. That forces females to prey on leaner market squid and rockfish, in effect, putting sea lion moms on a diet when they need to plump up to produce milk.

Researchers discussed the plight of sea lions during a conference last week at the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center.

Sea lion numbers exploded from 50,000 to more than 300,000 over the last 40 years. The surge in numbers marks a success for the 1972 Marine Mammal Protection Act, which made it illegal to harm or kill the animals, once hunted for their hides, meat and oil. But it leaves the population closer to its “carrying capacity” — the point at which food supply can curb their population, researchers said.

“The forage is down at the moment, the sea lion numbers are up, and the overall productivity of the system is down,” said Sam McClatchie, an oceanographer with NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, and lead author of the paper. “It’s a perfect storm. It’s an unfortunate natural phenomena.”

For the past decade, sea lion strandings averaged about 250 during the period between each January and April, said Justin Viezbicke with the National Oceanic and Atmospheric Administration’s fisheries service. In 2013, however, that shot up to 1,262, as skinny and sickened pups showed up on shore in droves, mainly between Santa Barbara and San Diego. Last year, strandings skyrocketed to an unprecedented high of 3,340.

The young animals were far smaller than normal, after weaning too early. Malnourished and dehydrated, many were barely over their birth weight of 18 to 22 pounds, their skin hanging loose on their bodies. Some were riddled with parasites, and plagued by respiratory infections and digestive maladies.

So far this year, California sea lion strandings are five times higher than the average level, but well below their 2015 peak, according to NOAA Fisheries. SeaWorld has rescued 326 sea lions so far this year, far less than the 800 animals they had rescued by the end of April last year, said spokesman Dave Koontz.

The Marine Mammal Care Center at Fort MacArthur in San Pedro reported that it rescued 246 sea lions between January and March of this year, compared to 411 in the same period in 2015.

The slowdown in strandings doesn’t necessarily signal an end to the crisis; researchers think there may be fewer animals showing up on shore because fewer pups were born, or made it off the islands alive.

Forage fish such as sardines and anchovies rise and fall periodically in normal but dramatic cycles. They tend to fall off during warm water periods that disrupt historical spawning times and locations. And the Pacific Ocean off the West Coast has been a fish sauna the past few years, with temperatures two to four degrees above normal.

That’s partially due to this year’s El Niño, a warming pattern around the equator that heats up waters off California. It also stems from other conditions including the Pacific Decadal Oscillation, a longer term temperature trend in the mid-Pacific, and to what oceanographers call “the blob” of persistent warm water off the West Coast. Those combined conditions drive down numbers of sea lions’ favorite fatty fishes.

“Our interpretation is that what’s going on at the moment is a natural phenomenon, not overfishing,” McClatchie said. “The trends result from natural variability of the forage fish population.”

Sea lions aren’t picky eaters, and consume at least 133 different species of prey. So when the oily little fish they favor drop off, they switch to other food. But all fish aren’t created equal, and many of the other options have less calories and nutrition than sea lions and their pups require.

On that alternative diet, the animals have to work harder to feed themselves and their young. Successful sea lion mothers took shorter, more frequent foraging trips, returning every two to three days to nurse their pups, said Sharon Melin, a research biologist for NOAA.

They also dove far deeper than usual – as far as 400 meters – in order to snag hake, a silvery fish related to cod and haddock. They have to expend a lot of energy on those plunging dives, making the food more costly to obtain.

Despite those efforts, the mothers deemed successful were pretty much just getting by, producing enough milk for their pups to survive, but not necessarily thrive.

“Some females are getting just enough to keep their pups alive,” Melin said.

While stranding numbers are down, the slowdown reflects the fact that fewer sea lions were born this year, said Jeff Laake, a statistician with NOAA. And it doesn’t count the ones that never make it to shore.

“We see more dying on the island,” Melin said. “They were in really poor condition in September, so a lot never made it off the island.”

As sad as it is to see individual animals suffering, Melin said it doesn’t signal a crisis for sea lion populations off the West Coast.

“We still have hundreds of thousands of sea lions in the population,” she said. “We’re not concerned that it’s catastrophic.”